Xenobiotic chemicals can be metabolized by organs and tissues other than the liver. Additionally, the prostaglandin endoperoxide synthetase (PES) system is found in a number of organs and is not dependent on cytochrome P-450. The metabolism of polycyclic aromatic hydrocarbons, aromatic amines, and other chemicals to mutagens in Salmonella tester strains was studied using PES and, also, microsomal preparations from rat lungs, testes, and prostate, in addition to liver.